Mechanics and diagnosticians of internal combustion engines have for many years used compression gauges in analyzing the condition and operation of cylinders, pistons and valves. Typically, compression gauges are of the manifold type and are pressed into the engine's spark plug aperture after the plug is removed, or in the case of diesel engines, into the injector-nozzle port after the injector nozzle is removed. Pressure from within the cylinder is then measured by cranking the engine and monitoring the pressure build-up on a needle gauge. In compression testing in the case of diesel engines, the injector pump is normally disabled during compression testing to avoid combustion with a nozzle removed. Similarly, the distributor is disabled during compression testing of carbureted engines to avoid combustion when the spark plug is removed. In either case, compression readings can only be safely taken at cranking speed.
While the pressure reading taken from an individual cylinder is of some value, it is generally recognized as an unreliable indicator of engine performance, particularly in simple compression gauges used in the field and by the consumer. More accurate and meaningful diagnosis normally requires that the ratings from all of the cylinders of the engine be read sequentially and compared to each other. Any reading lower than the norm by more than 10 or 20% is an indication of a malfunction within the cylinder, the malfunction normally being attributed to either worn or broken rings or to bad or broken valves.
It will be appreciated that analysis of the engine's performance with conventional compression gauges has several limitations. First, diagnosis in this manner requires removal of the plug from the cylinder, or in a diesel, the removal of the injector nozzle. This may be a very cumbersome, time-consuming task. Second, compression measured in this manner is static in nature and does not reflect the operation of the cylinder during regular operating conditions. Some engine faults cannot be detected from the compression readings during static conditions since they manifest themselves only during normal running of the engine and, in some instances, only when the engine is operating at high RPM. And obviously diagnosis with a compression gauge tells the user nothing about the condition of injector nozzles in a diesel engine if the nozzle must be removed during insertion of the gauge. Similarly, conventional compression testing reveals nothing with respect to the performance of spark plugs in a carbureted engine.
It has also been proposed that apparatus be provided for monitoring on a storage oscilloscope the pressure-related signals from multiple piezoelectric transducers of the type shown in Dooley U.S. Pat. No. 4,036,050.